Write and explain beta-minus decay equations
Track neutron-to-proton conversion and balance the emitted electron charge number.
Section 5.2.3 · Radioactive decay
Exam scope and assessment notes
Extended only (Supplement) · Physics 0972, 2026–2028. Use labelled nuclides, supplied count intervals and stated background conditions. Model drawings are not to scale. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Read signed charge labels.
- Know neutron and proton identities.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which nuclear conversion describes beta-minus?
What you will learn
- Track neutron-to-proton conversion and balance the emitted electron charge number.
- Explain and balance beta-minus equations while recognising their simplified particle scope.
Preserve nucleon number and change proton number
In beta-minus decay a neutron becomes a proton and an electron is emitted. The daughter A is unchanged, Z increases by one and neutron number decreases by one. The emitted electron is written ⁰₋₁e, so ¹⁴₆C → ¹⁴₇N + ⁰₋₁e balances 14 = 14 + 0 and 6 = 7 − 1.
The emitted electron arises in the nuclear change. It was not a pre-existing electron orbiting the atom that simply escaped. This makes beta decay different from ordinary positive-ion formation.
Recognise the simplified particle account
The syllabus equation balances nucleon number and charge number. In the fuller physical beta-minus process an antineutrino is also emitted; it has zero nucleon number and zero electric charge, so it does not change these two balances. The simplified equation is not a complete account of energy and momentum.
A may stay the same even though the element changes because Z identifies the element. The daughter may be more stable without that alone proving it is permanently non-radioactive.
Pause and explain
Why does the electron have A = 0 in a nuclide equation?
Worked example
Complete ¹⁴₆C → ? + ⁰₋₁e, given Z = 7 identifies N.
Show the worked solution
- Keep A = 14.
- Increase Z to 7.
- Check 7 − 1 = 6.
Answer ¹⁴₇N.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the syllabus beta-minus equation for ¹⁴₆C with daughter N.
Give me a hint
Keep A and balance Z with the emitted electron.
Compare my reasoning
- Daughter A is 14.
- Daughter Z is 7.
- The electron labels are 0 and −1.
¹⁴₆C → ¹⁴₇N + ⁰₋₁e.
Look for these in your work
- Balance 14 on both sides.
- Balance 6 = 7 − 1.
A parent has A = 24 and Z = 11. Find daughter A, Z and neutron number after beta-minus.
Give me a hint
Change one neutron into a proton.
Compare my reasoning
- A remains 24.
- Z becomes 12.
- Neutrons = 24 − 12 = 12.
A = 24; Z = 12; 12 neutrons.
Look for these in your work
- Keep nucleon count.
- Reduce neutron count by one.
A report says beta decay is a neutral atom losing an orbital electron. Correct the mechanism.
Give me a hint
Locate the changing particle in the nucleus.
Compare my reasoning
- A neutron becomes a proton.
- An electron is emitted in the nuclear change.
- Orbital-electron loss alone changes ion charge without this nuclear conversion.
Beta-minus is nuclear neutron-to-proton conversion, not ordinary ion formation.
Look for these in your work
- Locate the nuclear change.
- Distinguish ionisation.
Common mistakes
- Keep A unchanged.
- Beta electron is not orbital loss.
- Balance the negative electron label.
What can you explain now?
What is the charge number of a beta-minus electron?
Compare with the explanation
−1
The emitted electron carries one negative relative charge.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain and balance beta-minus equations while recognising their simplified particle scope.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.